This invention relates to two-component polyurethane coatings.
Currently-available two-component aliphatic polyurethane coatings offer an attractive combination of high gloss, excellent exterior durability, adequate hardness, abrasion resistance, and flexibility, as well as resistance to organic solvents, hydraulic fluids, and various other chemicals and stains. These coatings are easy to cure, because they can be cured at ambient temperature or with mild baking.
Because of these performance properties, polyurethane coatings are currently used for:
1. High performance industrial product finishes for aircraft and other large surfaces which cannot conveniently be baked because of their physical size, and also for wood, plastic, and other substrates which would be degraded or deformed by high temperature baking;
2. Industrial maintenance finishes which must be cured at ambient temperature, but require exterior durability, and solvent and chemical resistance equivalent to those of baked coatings; and
3. Linings for pipes and storage tanks which are required to handle solvents and corrosive chemicals.
A problem with most two-component polyurethane coatings and enamels available on the market is that they are applied by spraying at a volume solids content of less than 50%. The remaining portion of the coatings consists of petroleum-derived solvents. Pollution regulations, such as the Southern California Air Pollution Control District Rule 442, severely restrict the use of such polyurethane coatings, unless equipment is installed to reduce solvent emissions by at least 85%. Such solvent control equipment is very expensive. In some cases, the use of solvent control equipment is infeasible, such as the spraying of fixed structures, like the exterior of large storage tanks. However, regulations such as Regulation 442, contain more lenient requirements for coatings containing less than 20% by volume of organic solvents.
Since federal regulations and other local air pollution laws are likely to be modeled after those already in effect in California, it would be desirable to discover a high solids urethane coating which performs satisfactorily in the three above-mentioned applications. It is desirable that such coatings have application properties, cured film performance, and cost-per-square-foot per mil dry film thickness comparable to the presently-used low solids urethane coatings.
An approach which has been considered to produce high solids polyurethane coatings is to use a low-viscosity, polyol-containing component and a low-viscosity, isocyanate-containing component for producing the polyurethane coating. However, achieving a satisfactory low-viscosity polyol for use in polyurethane coatings is a difficult problem. A technique which has been evaluated is to lower the molecular weight of the polyol-containing component by combining monomeric polyols, such as ethylene glycol, propylene glycol, neopentyl glycol, hexylene glycol, 1,6 hexanediol, decylene glycol, glycerine, trimethylol ethane, trimethylol propane, and the like. These monomeric polyols can be reacted with low-viscosity monomeric diisocyanates and can be sprayed at 100% solids content. However, the coatings formed exhibit one or more of the following deficiencies:
1. Cost: Since the diisocyanate component is the most expensive part of the formulation, and it is added at least in stoichiometric amount relative to the polyol, the use of a polyol with a low-equivalent weight per hydroxyl group results in a very expensive completed coating. The above list of polyols have equivalent weights per hydroxyl group, varying from 31 for ethylene glycol to 87 for decylene glycol, and thus yield extremely costly polyurethanes.
2. Low flexibility: For sufficient flexibility in the polyurethane coating, some linear aliphatic hydrocarbon chains should be present in the polyurethane. Of the polyols listed above, only hexylene glycol, 1,6 hexanediol, and decylene glycol meet this requirement. The other polyols yield very brittle films with poor adhesion to steel.
3. Insufficient cross-linking: Cross-linking is required in the polyurethane coating to provide solvent and chemical resistance. When a glycol or a diol is reacted with a diisocyanate, the resulting polyurethane is primarily linear in nature. To achieve good chemical and solvent resistance properties with the diisocyanate, the polyol used should have a functionality greater than 2; that is, it should contain more than two hydroxyl groups per molecule to achieve a high level of cross-linking. The polyols listed above which contain more than 2 hydroxyls-glycerine, trimethylol ethane and trimethylol propane, yield costly and brittle polyurethanes.
Therefore, there is a need for a high solids polyol-containing component for producing a low cost polyurethane coating with good application properties and good cured film performance.